Effects of high- and low-intensity fires on soil properties and plant growth in a Bolivian dry forest

We compared soil nutrient availability and soil physical properties among f our treatments (high-intensity fire, low-intensity fire, plant removal, and harvesting gap) and a control (intact forest understory) over a period of 18 months in a tropical dry forest in Bolivia. The effect of treatments on plant growth was tested using a shade intolerant tree species ( Anadenanthe ra colubrina Vell. Conc.) as a bioassay. Surface soils in high-intensity fi re treatments had significantly greater pH values, concentrations of extrac table calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P), and amounts of resin-available P and nitrogen (N) than other treatments; howeve r, a loss of soil organic matter during high-intensity fires likely resulte d in increased bulk density and strength, and decreased water infiltration rates. Low intensity fires also significantly increased soil pH, concentrat ions of extractable Ca, K, Mg, and P, and amounts of resin-available P and N, although to a lesser degree than high-intensity fires. Low-intensity fir es did not lower soil organic matter contents or alter soil physical proper ties. Plant removal and harvesting gap treatments had little effect on soil chemical and physical properties. Despite the potentially negative effects of degraded soil structure on plant growth, growth of A. colubrina seedlin gs were greater following high-intensity fires. Evidently, the increase in nutrient availability caused by high-intensity fires was not offset by degr aded soil structure in its effects on seedling growth. Long-term effects of high intensity fires require further research.